Manufacturing of semiconductor devices commonly requires deposition of electrically conductive material on semiconductor wafers. The conductive material, such as copper, is often deposited by electroplating onto a seed layer of metal deposited onto the wafer surface by a PVD or CVD method. Electroplating is a method of choice for depositing metal into the vias and trenches of the processed wafer during Damascene and dual Damascene processing.
Damascene processing is used for forming interconnections on integrated circuits (ICs). It is especially suitable for manufacturing copper interconnections. Damascene processing involves formation of inlaid metal lines in trenches and vias formed in a dielectric layer (inter-metal dielectric). In a typical Damascene process, a pattern of trenches and vias is etched in the dielectric layer of a semiconductor wafer substrate. A thin layer of diffusion-barrier film such as tantalum, tantalum nitride, or a TaN/Ta bilayer is then deposited onto the wafer surface by a PVD method, followed by deposition of seed layer of copper on top of the diffusion-barrier layer. The trenches and vias are then electrofilled with copper, and the surface of the wafer is planarized to remove excess copper.
The vias and trenches are electrofilled in an electroplating apparatus, such as the SABRE™ clamshell electroplating apparatus available from Novellus Systems, Inc. of San Jose, Calif., and described in U.S. Pat. No. 6,156,167, which is incorporated herein by reference in its entirety. Electroplating apparatus includes a cathode and an anode immersed into an electrolyte contained in the plating vessel. The cathode of this apparatus is the wafer itself, or more specifically, its copper seed layer and the deposited copper layer. The anode may be a disc composed of, e.g., phosphorus-doped copper. The composition of electrolyte that is used for deposition of copper may vary, but usually includes sulfuric acid, copper salt (e.g. CuSO4), chloride ions, and a mixture of organic additives. The electrodes are connected to a power supply, which provides the necessary voltage to electrochemically reduce cupric ions at the cathode, resulting in deposition of copper metal on the surface of the wafer seed layer.
The composition of plating solution is selected so as to optimize the rates and uniformity of electroplating. Copper salt serves as a source of plated copper and also provides conductivity to the plating solution. Sulfuric acid enhances plating solution conductivity by providing protons as current carriers, and, therefore, allows electrodeposition of copper at reduced applied voltages. Organic additives, known as accelerators, suppressors and levelers, are capable of selectively enhancing or suppressing rates of deposition of copper on different surfaces of the wafer features, thereby improving the uniformity of deposition. Chloride ion is useful for modulating the effect of organic additives and is commonly added to the plating bath for this purpose.
It is often advantageous to separate anodic and cathodic regions of the plating cell by a membrane because processes occurring at the anode and the cathode during electroplating are not always compatible. For example, during use, insoluble particles resulting from flaking of the anode, or from precipitation of inorganic salts may be formed at the anode. It is desirable to protect the wafer from these particles, so that they would not interfere with the metal deposition process and would not contaminate the wafer. In another example, it may be desirable to confine organic additives to the cathodic portion of the plating cell, so that they would not contact the anode. Organic additives used for modulation of deposition rates often contain thiol groups and are prone to oxidative decomposition at the anode surface, resulting in anode passivation.
A suitable separating membrane would allow the flow of ions, and, hence the current, between the anodic and cathodic regions of the plating cell, but will block larger particles, and some non-ionic molecules, such as organic additives from crossing it. By doing so, the membrane essentially will create different environments in the cathodic and the anodic regions of the plating cell. The isolated anodic region of the plating cell is often referred to as a separate anode chamber (SAC) and electrolyte within it is known as anolyte. The electrolyte contained in the plating bath across the membrane from the SAC is referred to as catholyte.
Electroplating apparatus having membrane-separated cathode and anode chambers achieves separation of catholyte and anolyte and allows them to have distinct compositions. For example, organic additives can be contained within catholyte, while the anolyte can remain essentially additive-free. Further, anolyte and catholyte may have differing concentrations of copper sulfate and sulfuric acid, due, for example, to ionic selectivity of the membrane. An electroplating apparatus having a membrane is described in detail in U.S. Pat. No. 6,527,920 issued to Mayer et al., which is herein incorporated by reference for all purposes.
The membrane separating catholyte and anolyte may have different selectivity for different cations. For example, it may allow passage of protons at a faster rate than the passage rate of cupric ions. During electroplating, the current can be carried between the anode and the cathode by any cationic species, e.g. by both protons and copper ions. However, depending on the selectivity of the membrane, mobility of the ions or other factors, the current may be predominantly carried by protons, until a certain molar ratio between Cu2+ and H+ concentrations is achieved. After this ratio is achieved, copper ions start crossing the membrane and carrying the current along with the protons. Therefore, until a certain molar ratio between copper ions and hydrogen ions is achieved, the anolyte is being continuously depleted of its acidic component, since the protons are the main current carriers under these conditions. While concentration of acid in the anolyte is being continuously decreased, the concentration of copper salt is increased, especially when a copper-containing anode is used.
These processes may result in several undesired effects in the plating system. First, if solubility limit of copper salt is reached before cupric ions start carrying the current and start leaving the anolyte, the copper salt would precipitate in the anode chamber. This salting out may cause passivation of the anode, which is characterized by deposition of copper salt on the anode surface. Clogging of filters in the anolyte recirculation loop is also occurring as a result of copper salt precipitation.
Further, the separation of cathodic and anodic regions by a membrane creates an electroosmotic effect in which the protons crossing the membrane from the anode chamber to the cathodic portion of the apparatus “drag” water molecules in the same direction thereby depleting the anolyte volume and increasing the volume in the cathode chamber. This effect is known as electroosmotic drag and is undesired since it creates a pressure gradient between the two chambers that can lead to membrane damage and failure.
The salting out effect can be alleviated to some extent by replenishing the anolyte continuously with the fresh electrolyte and by disposing of or reconstituting the old electrolyte that has high copper salt concentration. This method is known as bleed and feed method. While it is generally desirable to refresh small percentage of anolyte by bleed and feed method, it is not an economically feasible method for solving copper salt precipitation problem. High bleed and feed rates are generally needed to maintain acceptable copper concentration in the anolyte, resulting in large volumes of electrolyte being wasted. Therefore, operation cost of electroplating apparatus becomes very high when high bleed and feed rates are used.
It is desirable to be able to control composition of the electrolyte in a more economical fashion. Accordingly, a method of such control, and an apparatus allowing practice of such a method, are needed.